For many military and commercial missions it is desirable to fly an aircraft at very high altitude, ideally with the capability to stay on station indefinitely. The missions that benefit from this capability are those that take advantage of the long line-of-sight to the horizon enjoyed by such a high altitude platform. Missions with both military and commercial utility include surveillance and communications connectivity. A high altitude, long endurance aircraft also has application to the military signals intelligence mission. Finally, a high altitude, long endurance aircraft has application to space exploration, inasmuch as such a vehicle can be flown in any type of planetary atmosphere. An aerodynamic design that can operate at the very top of the Earth's atmosphere is naturally well-suited to flying at a lower altitude on a world with low atmospheric density at the planetary surface.
Alternatives to high altitude, long endurance aircraft include orbiting satellites and those aircraft that can achieve high altitude but without the capability for long endurance at that altitude. Both of these alternatives have operational and cost disadvantages. Satellites are expensive to launch and operate and, except in the case of geostationary satellites, cannot loiter over a point on the ground. Consequently, large constellations of satellites are required for global coverage or to ensure high revisit rates (i.e. short gaps in coverage) with respect to a ground target or ground station. Geostationary satellites remain fixed with respect to a location on the ground only when launched into an orbital slot above the equator, which drastically limits ground coverage, especially at high latitudes. Finally, aircraft without long endurance at high altitudes have inefficiencies of operation, owing to the need to cycle such vehicles back and forth from a launch base to a mission station (e.g. the locus of the surveillance, communication, or other mission activities in question). For example, at any given time one aircraft might be flying to relieve a second aircraft on-station, while a third is flying back to the launch base from the station, and a fourth is at the launch base being prepared for takeoff. The required fleet size, and thus the overall cost, increases with the distance of the mission station from the launch base. Furthermore, along the entire flight path of the cycling aircraft, the operation becomes subject to the vagaries of nature (e.g. storms) and, in the military case, enemy action, etc.
As a result of the cost and operational disadvantages of alternatives, a viable high altitude long endurance aircraft has become something of a holy grail for aircraft designers. Furthermore, for reasons of operational responsiveness, it is desirable that such an aircraft be rapidly deployable to a distant operating location without being impeded by adverse weather conditions. Satisfying these requirements with one design is an extremely difficult technical challenge.
For reasons of less-than-perfect subsystem reliability, no high altitude long endurance aircraft that could operate on-station indefinitely has heretofore been conceived. In the past, it has been recognized that it is possible to design aircraft, the endurance limits of which are not bounded by the supply and consumption of onboard fuel. Such an aircraft could maintain a mission station at an altitude for perhaps several months, until subsystem failures forced a return to base. There are three general cases: nuclear propulsion using an onboard nuclear fission reactor, power beamed to the aircraft from the ground (e.g., using microwaves or laser energy), and solar-electric propulsion.
The United States explored nuclear-powered aircraft in the 1950s, but the effort that involved a modified Convair B-36 Peacemaker test-bed aircraft and ground-based test articles was terminated. It is highly unlikely that contemporary environmental awareness and political sensitivities would allow a similar concept to be pursued today.
Small remote-controlled aircraft that are powered by means of energy beamed from a ground site have been designed and, in some cases, flown. Effectiveness is limited by very poor efficiencies when distance from the ground site becomes large, as a consequence of beam spreading and the resulting reduced energy flux received by the aircraft. Furthermore, if beam spreading is minimized by resorting to higher frequencies of energy transmission, flux is improved at the cost of increased environmental risk (e.g. birds and other aircraft may fly through the beam at intermediate altitudes). The practical result of these limitations is that the beam-powered aircraft is virtually tethered close to its source of power, which is operationally undesirable in most cases.
Solar-electric propulsion is the third pathway to effectively unlimited flight and, in fact, full-scale unmanned and manned solar-electric airplanes have been flown. Examples include the Aerovironment Pathfinder and Helios. Reliance on solar flux causes solar-electric aircraft designs to have very low propulsive power, which in turn places a premium on aerodynamic and structural design. Furthermore, such aircraft are best operated at very high altitude, ideally more than 50,000 feet above sea level, to ensure that clouds do not reduce received solar flux and to minimize the chance of encountering headwinds.
As a result of these considerations, current efforts to achieve a “forever on-station” high altitude aircraft have largely focused on solar-electric aircraft. There are two types of aircraft under consideration: heavier-than-air aircraft (e.g. airplanes) and lighter-than-air aircraft (e.g. airships). Airships derive their lift from aerostatic means (e.g. from a buoyant force provided by a lifting gas such as helium) rather than from aerodynamic forces acting on a wing. A solar-electric airship currently under development is the Lockheed-Martin High Altitude Airship.
In both airplane and airship cases, the combination of low power (which is due to the limits of solar flux) and high altitude results in the need for very large, lightweight structures. In the airplane case, wing loading (i.e. the ratio of airplane weight to wing area) must be very low. In the airship case, hull fabric weight per surface area must be very low. Consequently, both airplanes and airships will be relatively fragile. Additionally, airspeeds of both types of vehicle will be very low due to the low power that is available. These aircraft are consequently at risk of catastrophic structural failure or being blown uncontrollably downwind, as a result of gusts or high winds respectively, while climbing or descending through the lower atmosphere or while being launched.
The most efficient aerodynamic configuration in terms of lift-to-drag ratio for a high altitude solar-electric aircraft is that of a high aspect ratio unswept flying wing, where aspect ratio is defined as the square of wing span divided by reference wing area. “Flying wing” refers to an airplane that is comprised of a wing alone, without fuselage or empennage. This was, in fact, the configuration of the Aerovironment Pathfinder and Helios aircraft. The primary aerodynamic disadvantage of such a configuration is that stability and control are inherently poor, especially in the longitudinal or pitch sense, since with no tail surfaces there can be no significant tail moment arm. The primary structural disadvantage of the lightweight, high aspect ratio flying wing configuration is that there can be little resistance to span-wise bending and little torsional stiffness (i.e. resistance to wing twisting). In particular, in the solar-electric case, there is no fuel carried in the wing, the weight of which would serve to react against the first wing bending moment. If payload is not distributed across the span of the wing (i.e. span-loaded) but is instead concentrated at the centerline of the vehicle, the problem of span-wise bending is aggravated. Finally, these aerodynamic and structural difficulties can combine in the form of aero-structural interactions—for example, the aircraft can develop wing flapping and twisting oscillations that cause uncontrollable and potentially divergent oscillations in flight path. This sequence of events was, in fact, the proximate cause of the in-flight breakup of the Aerovironment Helios over the Pacific Ocean in 2003.
Returning to the airship case, the lightweight fabrics required for high altitude airship flight are problematic. For reasons of weight, high altitude airships must be of non-rigid design, where hoop stresses and hull bending moments are carried by the hull fabric alone. Such fabrics must also resist tearing, resist ultraviolet radiation, and be very impermeable to helium. Historically, hull structural failure of airships operating at low altitude has been a recurring difficulty, and the requirement for lightweight fabrics at high altitude makes matters worse. Finally, to carry a reasonable payload, the high altitude airship must be extraordinarily large, on the order of 500 feet in length or more. This limits basing opportunities and introduces ground handling difficulties.
The exemplary embodiments described herein incorporate the premise that the technological and programmatic risks associated with high altitude airships are greater than those of high altitude airplanes, and proposes a solution for the aero-structural limitations of high aspect ratio flying wing airplanes. This solution entails subdivision of the wing into autonomous modular units that can join together in-flight, wingtip-to-wingtip, forming a single, multiple-articulated flying surface of great aerodynamic efficiency. A preferred embodiment includes a modular articulated-wing aircraft as above, with a solar-electric power system to provide motive force and satisfy mission system and housekeeping electrical demands.
There are in principle two ways of arranging low aspect ratio wing elements so as to approximate the aerodynamic efficiency of a higher aspect ratio wing. The first is, as above, to join the wing elements at the wingtips, creating an actual continuous wing surface. The second approach is to form a virtual wing, where wing elements are arranged in a chevron as seen from above, akin to the arrangement of a flock of geese flying in formation. In this latter case, aerodynamic benefits accrue from trailing wing elements being positioned precisely in the upwash of the element in front—in effect, the wing element is hitching a ride on the preceding element. In theory, the virtual wing approach can lead to impressive gains in aerodynamic efficiency, and since the wing elements are physically isolated there is no difficulty with wing bending. However, there are practical difficulties. The relative positions of wing elements must be very precisely controlled—the vorticity of airflow behind a wing means that a slight shift in lateral positioning can result in a wing element being in the downwash rather than upwash of the preceding element. There must be constant rotation in the positions of elements in the virtual wing, as the lead element gets no “free ride” and must periodically fall back, as does the lead goose in a flight of geese. Finally, and perhaps most seriously, aerodynamic modeling of such a virtual wing is difficult and the net aerodynamic benefits of the configuration currently are speculative.
The concept of aircraft joined at the wingtips to improve aerodynamic performance is not new. However, the prior art is restricted to aircraft of unequal sizes joined with the advantages of improving range and endurance rather than identically-sized aircraft joined with the advantage of attaining high altitude. Generally, small “hitchhiker” aircraft attach themselves to the wingtips of a much larger “mothership” aircraft (e.g. fighters attached to the wingtips of a bomber), enabling the hitchhikers to cover long distances that would otherwise be beyond their capability. Meanwhile, thanks to the aerodynamic advantage of an effectively higher aspect ratio wing, the mothership incurs little or no fuel consumption penalty.
The United States Air Force conducted flight tests of hitchhiker-mothership compound aircraft beginning in 1949. The objective was to demonstrate the capability for intercontinental bombers to be escorted for thousands of miles to their targets and back, and this was only possible if the fighters were carried or assisted by the bombers in some fashion. From 1949 to 1950, flight tests of a wingtip-linked Douglas C-47A transport and a Culver Q-14B trainer were conducted. These tests were promising, and were followed by tests of a Boeing B-29 Superfortress bomber linked at the wingtips to two Republic F-84 jet fighters in a project designated “Tip Tow.” Unfortunately the B-29 and one of the F-84s were lost with all souls in 1953. An automatic flight control system whose purpose was to control flapping angle failed to function as expected, and the doomed F-84 rotated about the wingtip connection, impacting the wing of the B-29. Flapping angle is defined as the angle between the wings of two joined aircraft in the lateral direction.
Another Air Force tip-docking project designated “Tom Tom” was conducted from 1952 to 1953. The Tom Tom project flight tested a Convair B-36 Peacemaker bomber attached at the wingtips to two F-84 fighters. On a test flight in late 1953, an uncontrollable oscillation developed between the B-36 and one of the F-84s, and the B-36 suffered major damage to its wing. The F-84 returned to base with a large section of the B-36's wing structure still attached to its wingtip.
As a result of these difficulties, Projects Tip Tow and Tom Tom were cancelled, and the Air Force ceased further experimentation with tip-docking compound aircraft concepts. The technology of the time was deficient in a number of areas. It was difficult or impossible to analytically predict complex flow fields and the interactions of flexible, linked aero-structures. It was an enormous challenge to design the automatic flight control systems that were necessary for tip-linked operations. Note that the hitchhiker-mothership type of compound aircraft has inherent difficulties that are not a feature of compound aircraft comprising multiple, small, equal-sized flight elements. Specifically, the mothership is large and heavy relative to the hitchhikers, and consequently the hitchhikers contend with very strong trailing wingtip vortices generated by the mothership. These vortices become a hazard during docking or undocking maneuvers.
In 2002, a doctoral dissertation by S. A. Magill titled “Compound Aircraft Transport Study Wingtip-Docking Compared to Formation Flight” was published by Virginia Polytechnic Institute. This document outlined a technical investigation of the hitchhiker-mothership type of compound aircraft in tip-docked and formation flight modes. The latter mode involves the creation of a virtual wing in chevron as discussed in the preceding text. The document did not consider the tip-docking of multiple, equal-sized aircraft. It did not address the pursuit of any type of compound aircraft design for the purpose of improving aircraft ceiling or performance at high altitude.
Thus, it will be appreciated that there is a need in the art to overcome one or more of these and/or other disadvantages. It also will be appreciated that there is a need in the art to provide a viable high altitude long endurance aircraft.
In certain exemplary embodiments, an autonomous modular flyer operable to loiter over an area of interest at a first high altitude is provided. Such flyers may comprise an airborne object having two wings, with each wing having a wingtip, and the wingtips being operably joinable to at least one other autonomous modular flyer's wingtips to form an aggregation when a first predetermined condition is met, and being operably disaggregable from the at least one other autonomous modular flyer's wingtips when a second predetermined condition is met. The aggregation may form a multiple-articulated flying system having a high aspect ratio wing platform, operable to loiter over the area of interest at an altitude at least as high as the first high altitude.
Autonomous modular flyers and/or aggregations thereof may be further operable to match their airspeed to a prevailing headwind and/or to make large orbits. Autonomous modular flyers and/or aggregations thereof may have an altitude ceiling in Earth's stratosphere and/or structural robustness in Earth's troposphere. The autonomous modular flyer may further comprise a wingtip hinge on at least one wingtip allowing two operably joined modular flyers to flap about the wingtip hinge with respect to each other.
Aggregations of larger numbers of modular flyers may occur at sequentially higher altitudes. A second predetermined condition may include one or more of: a loading event above a given load threshold, a gust above a gust threshold, a turn of the multiple-articulated flying system, a span shear above a span shear threshold, an instruction for at least one of the modular flyers to undertake a remote surveillance activity, and an instruction for at least one of the modular flyers to move closer to the area of interest. The multiple-articulated flying surface of claim 1 may be operable to reaggregate based at least on a third predetermined condition. That third predetermined condition may include one or more of: a second predetermined condition that previously was met no longer is met, and at least one modular flyer being destroyed, recalled, and/or no longer functional.
Insolation circuitry may power each modular element and/or the multiple-articulated flying system, and the insolation circuitry may comprise a photovoltaic array, an electronic controller to condition and manage the power, and an electrical energy storage mechanism. A flight controller operable to calculate an equilibrium ceiling altitude and to instruct the autonomous modular flyer to climb or descend to the equilibrium ceiling altitude may be included in modular flyers.
Certain exemplary embodiments provide a method of forming a multiple-articulated flying system having a high aspect ratio wing platform, operable to loiter over an area of interest at a high altitude. Such methods may comprise providing at least two autonomous modular flyers, with each having two wings with wingtips thereon. The wingtips of the at least two autonomous modular flyers may be joined when a first predetermined condition is met.
Such methods may further comprise calculating an equilibrium ceiling altitude for the autonomous modular flyer, and altering the autonomous modular flyer's altitude to the equilibrium ceiling altitude. Also, an equilibrium ceiling altitude for the multiple-articulated flying system may be calculated, and the multiple-articulated flying system's altitude may be altered to match the equilibrium ceiling altitude.
Also, data related to the area of interest may be sensed by an individual modular flyer. When a multiple-articulated flying system is formed, data may be shared between sensors of modular flyers and/or using individual sensors of modular flyers as elements in a sensor array.